1. Field of the Invention
This invention relates to a method for preparing electrically conductive polyorganosiloxane compositions. More particularly, this invention relates to a method for increasing the electrical conductivity of moisture curable organosiloxane compositions containing light-colored electrically conductive fillers without adversely affecting the properties of the cured compositions. This is achieved by defining the order in which the ingredients, particularly the curable polyorganosiloxane, filler and curing agent are blended with one another.
2. Background Information
Clean rooms are used in the manufacture of electronic devices and in the fabrication of the semiconductors devices, including integrated circuits, that are the heart of such devices. The purpose of a clean room is to prevent dust adhesion and discharge failure due to static electricity. Materials resistant to the accumulation of static electrical charges are used for the walls and floors of clean rooms, and electrically conductive silicone sealants are used for their joints and seams.
Prior art electrically conductive silicone sealants typically consist of room-temperature-curable organosiloxane compositions containing fillers such as carbon black and silver powder to provide the required electrical conductivity. These fillers impart a black or other dark color to the composition, which contrasts with the white or light colors of the wall materials used in clean rooms and makes these rooms less aesthetically attractive.
Room temperature curable organosiloxane compositions containing white or lightly colored electrically conductive fillers such as titanium oxide whose surface has been treated with a blend of antimony and tin oxides are known, however this type of organosiloxane composition does not exhibit a stable or consistent conductivity. More specifically, a drawback to this type of composition is that it does not develop a satisfactory conductivity unless the conductive filler is added in large quantities. This causes a high viscosity with the associated poor workability.
Japanese Laid Open Patent Application Number 1-287169 [287,169/1989] teaches imparting a stable conductivity to room temperature curable organosiloxane compositions containing light colored electrically conductive fillers by diluting the compositions with an organic solvent. While the solvent reduces the viscosity of the curable composition and improves the dispersibility of the conductive filler, the composition undergoes volumetric shrinkage upon curing. As a consequence, when such a composition is used in the joints and seams of wall or floor material, its performance is unsatisfactory because it has a poor sealing performance.
One objective of the present invention is to provide a method for preparing conductive organosiloxane compositions containing relative high concentrations of light-colored electrically conductive fillers that have excellent workability in the uncured form and cure to yield elastomeric products with excellent electrical conductivity and physical properties.